Late-Onset Alzheimer's disease (LOAD) is biochemically characterized by abnormal elevations of AB peptide and increased tau phosphorylation. Recently, reduced activity ofthe Retromer complex, which is important for the recycling of transmembrane receptors from endosomes to the Trans-Golgi Network (TGN), has been implicated in the pathology of LOAD from human patient expression profiling. The importance of retromer trafficking to LOAD is supported by several studies including both mouse and Drosophila genetic models of retromer deficiency, which have increased levels of Ap peptide, neurological deficits, and in the fly, extensive neurodegeneration. Defective retromer trafficking also inhibits Wnt signaling, suggesting a pathway via glycogen synthase kinase 3 beta (GSKSp) through which retromer could alter tau phosphorylation. We hypothesize that defective retromer sorting is central to both elevated AB peptide levels and increased tau phosphorylation in LOAD and that modulating retromer trafficking levels will have a positive impact on neurodegeneration. We will test this hypothesis in transgenic Drosophila models of LOAD where human Amyloid Precursor Protein (APP) and Amyloid Precursor Protein li-secretase (BACE) or human Tau are expressed. Our specific aims are designed to determine the molecular pathway that connects retromer deficiency to neurodegeneration and characterize novel interacting proteins that could promote retromer stability.